HSP27 and αB-crystallin localize to the I-bands in adult but not fetal rat cardiomyocyte cultures. (A) Schematics of the titin isoform sizes in fetal and adult rat cardiac sarcomeres. (B) Typical immunofluorescence images of fetal rat cardiomyocytes staining for HSP25/27 (secondary antibody: FITC-conjugated IgG) or endogenous αB-crystallin (secondary antibody: FITC-conjugated IgG; left) and counterstaining for A-band marker myosin-binding protein C (MyBP-C) (secondary antibody: Cy3-conjugated IgG) and Z-disk marker α-actinin (secondary antibody: Cy3-conjugated IgG), respectively. (C) Typical immunofluorescence images of adult rat cardiomyocytes staining for endogenous HSP25/27 (secondary antibody: Cy3-conjugated IgG) or endogenous αB-crystallin (secondary antibody: FITC-conjugated IgG) and counterstaining for A-band marker MyBP-C (secondary antibody: FITC-conjugated IgG) and Z-disk marker α-actinin (secondary antibody: Cy3-conjugated IgG), respectively. (insets) Higher-power images of the regions of interest indicated in the main panels. Arrows point to a doublet indicative of I-band binding of the sHSPs. Right panels show merged images. Rat HSP25 is homologous to human HSP27. Bars: (main) 10 µm; (insets) 2 µm.